Unravelling the performance of the SHARK-NIR Four Quadrant Phase Mask in a controlled environment

SHARK-NIR is an instrument that is mounted on one of the two arms of the Large Binocular Telescope (LBT) and it concluded its commissioning phase in October 2023. The instrument is dedicated to the infrared bands Y, J and H and it is equipped with several coronagraphic masks. This work is focused on the Four Quadrant Phase Mask (FQPM). The FQPM is characterized by a four quadrant pattern with the two adjacent quadrants providing a π phase shift in order to produce destructive interference with the stellar light. The FQPM is placed at the first focal plane (coronagraphic plane) while a Lyot Stop is located at the second pupil plane of the instrument. The FQPM, when tested on sky, did not reach the expected performance. For this reason, in the laboratory of the Astronomical Observatory of Padova, a test bench has been realized to investigate the causes and the possible solutions. This test bench has been built in order to replicate exactly the optical properties and the working wavelength (λ = 1.6 µm) of the FQPM of SHARK-NIR (actually the FQPM is a spare part of SHARK-NIR) and with the additional possibility to analyze the quality of the beam on the coronagraphic plane. The controlled environment of the laboratory allowed us to test different configurations in the first pupil plane of the test bench, including: a 2-inch plane mirror, a 1-inch mirror with Tip/Tilt motors and an ALPAO 97-15 deformable mirror (DM) (the same model mounted on SHARK-NIR). The Phase Diversity (PD) technique has been used at the coronagraphic plane and at the science plane to analyze the optical quality of the beam and to tackle the problem of the Non Common Path Aberrations (NCPA). The configurations used in the laboratory, the procedures to align the FQPM and the results are reported in this work.